The present invention relates to furnaces of the type employed for heating a sample positioned in a resistance crucible held between a pair of electrodes.
In resistive furnaces used as part of an analyzing instrument, a pair of opposed electrodes is provided, between which there is positioned an electrically resistive, graphite crucible into which a specimen to be analyzed is inserted. During operation, one of the electrodes is moved toward the other so that the crucible is clamped between the two electrodes. When electrical current is passed through the clamped crucible, its temperature rises to approximately 2500.degree. C., thereby fusing the sample. An example of such a furnace is shown by U.S. Pat. No. 3,936,587, issued Feb. 3, 1976, to Sitek et al and assigned to the present assignee.
During the course of running specimens through the furnace, the electrodes become contaminated by the byproducts of combustion. It, therefore, becomes necessary after the running of a few specimens to clean the contaminated electrode surfaces. To facilitate cleaning, an upper electrode has been positioned within a demountable housing. This housing can then be removed to facilitate cleaning, such a device being disclosed in U.S. Pat. No. 4,056,677, issued Nov. 1, 1977, to Berk et al and assigned to the present assignee.
Present furnaces make use of disc-like electrode inserts made up of an electrically conductive, wear resistant material for supporting the crucible in the furnace. These inserts are silver soldered to the crucible contacting surfaces of the electrodes. This construction is also disclosed in the above identified U.S. Pat. No. 3,936,587. The lower insert has, with continued use, a tendency to fail by flaking, chipping or cracking due to the arcing of electrical current between the crucible bottom and the insert. When the worn or damaged insert is required to be replaced the use of the furnace is delayed while the insert is removed and resoldered. In some furnaces, the entire lower electrode can be removed, however, removal of this electrode requires time and breaks the integrity of the electrode liquid cooling system.